1. Field of the Invention
The present invention relates to a gas concentration detecting device for detecting the concentration of a particular gas component in a gas to be detected. In particular, the present invention relates to a gas concentration detecting device suitable for detecting the concentration of a gas such as oxygen in an exhaust gas of an internal combustion engine.
2. Description of Related Art
A known gas concentration detecting device is shown in FIG. 1. The device shown in FIG. 1 is mounted to a wall of an exhaust pipe of an internal combustion engine and used for detecting the concentration of oxygen in an exhaust gas in an exhaust gas recirculating (EGR) system for an internal combustion engine to allow a feedback control of a recirculated exhaust gas (EGR gas) in accordance with the detected concentration of the gas. The gas concentration detecting device includes a housing 92 having a flange 91 which is connected to a wall of an exhaust gas of an internal combustion engine. Inside the housing 92, a gas concentration detecting body 93 of a tubular shape is stored, so that a lower half part of the body 93 is projected downwardly from the housing 92 toward the space inside the exhaust pipe P, while being encircled by a cover unit 94. The gas concentration detecting body 93 is formed as a tubular member made of a solid electrolyte such as a zirconia having an inner and outer surfaces on which electrodes made from a material such as a platinum are formed. Furthermore, a heater 95 is arranged in the gas concentration detecting body 93 for heating the latter so that the precision of detection is increased.
The cover unit 94, which is to allow the gas concentration detecting body 93 to be thermally insulated and prevents the body 93 from being subjected to a mechanical impact force, is formed as a double tube structure including an inner tube 94a and an outer tube 94b. These tubes 94a and 94b are, at their cylindrical walls, formed with gas vent holes 941 and 942 in an alternating manner, so that the speed of the inflow of the exhaust gas is reduced, thereby preventing the electrodes from being degraded. As is well known, in a diesel engine, a temperature of the exhaust gas is reduced and an amount of the exhaust gas is increased in comparison with the gasoline internal combustion engine. As a result, in the diesel engine, the gas concentration detecting body may be quickly cooled if contact between the exhaust gas and the gas concentration detecting body 93 occurs at the outer surface of the body. The cover 94 is designated to prevent such a cooling of the body 93 as much as possible.
Namely, the arrangement of the cover 94 is such that it encircles the end of the gas concentration detecting body 93, which is in the flow of exhaust gas, thereby maintaining the temperature of the body 93.
However, in the structure of the gas concentration detecting device in the prior art, as shown in FIG. 1, the gas vent holes 941 and 942 in the peripheral wall of the cover 94 are opened in the direction of the flow of the exhaust gas. As a result, the flow of the exhaust gas is able to pass through the cover without being subjected to a substantial speed reduction. As a result, in a diesel engine where the gas concentration detecting device having the above mentioned cover unit 94 is employed, it its likely that a heat retaining capacity of the gas concentration detecting body 93 due to the cover unit 94 is not sufficiently high so that the temperature of the surface of the gas concentration detecting body 83, as well as the temperature of the cover unit 94 itself, are low which may cause particulates in the exhaust gas in the diesel engine to be attached or deposited on the surface of the cover. These particulates includes components such as soot and soluble organic fraction (SOF). The SOF has an increased value of a viscosity, which may cause the soot to be attached to the surface of the cover unit 94 via the SOF, when the temperature of the surface is low, due to the fact that the lower temperature causes the SOF to be less volatile. Such attached and deposited particulates are partly subjected to combustion by contacting high temperature exhaust gas during a high load operation of the engine, which causes the composition of the exhaust gas, i.e., an oxygen concentration to be varied in the atmosphere around the gas concentration detecting body 93, which results in an error in the oxygen concentration measurement. Furthermore, the gas vent holes 941 and 942 are likely to be clogged by particulates, which causes the speed of the passage of the gas to be reduced, thereby reducing measuring response speed and generating a measuring error. In order to obviate these problems, a solution for increasing the heat generated by the heater can be employed. However, this solution necessitate an increased load in the heater and an increased electric power consumption by the heater.
Japanese Utility Model Publication No. 53-103784 proposes a oxygen concentration sensor, wherein a cover has a closed end of an increased heat mass, which can increase a heat retaining property with respect to a change in the temperature of the exhaust gas. However, in this construction the cover unit has, at its side wall, gas vent holes which cause the above problems to still be generated.
In view of the above problems, the inventors conceived a solution in that the cover unit is, at its side wall, free from any gas passage hole, while gas passage holes are formed at tip end of the cover unit, so that the amount of the exhaust gas in contact with the gas concentration detecting body is reduced, thereby preventing an occurrence of a flow of the exhaust gas transverse the cover, thereby preventing the cover unit from being overly cooled. U.S. Pat. No. 5,073,247 discloses such a construction of a cover unit having a gas vent hole only at end wall of the cover, i.e., no provision of vent hole at side wall of the cover, although the subject of this prior art is not at all related to a cooling of the gas concentration detecting body and the cover. However, in this patent, only one gas passage hole is provided at the tip end of the cover, so that an inflow of the gas to the space inside the cover and an outflow of the gas from the space are occurred simultaneously via the single hole, which causes a gas ventilation to be not smooth between the inflow and outflow of the gas, thereby reducing the response speed in a measurement.